52. Concerning the Conformation of Isolated Benzylideneanilinel)

(15. IX. 75) Sumvnavy. From PE.-spectroscopical studies the torsional angle p, of the N-phcnyl ring in isolated benzylideneaniline 1 has been found to be dcfinitely srnallcr than p, = 90'. An ap-proximate value q ~ = 36 " has been estimated which is ewn sniallcr than t h e one observed in the crystal (p, = 55") and suggested to prevail also in solutions of 1. 11 reevaluation of the gas phase optical spectrum of isolated 1 supports a torsional angle similar to that found in the other phases. Calculations of the most stable conformation of 1 as \re11 as of stilbene and azobenzene by the MINDO/3-tcchnique lead to torsional angles p, = 90 " for both phenyl rings in all cases. These results are at variance with the experimental results ant1 suggcst t?iat MiND0/3-like its less advanced precursor MINDO/2 or likc CND0/2-is unreliable lor low cncrgy 12rocesscs involving rotation of n-systems connected by esscntial singlc bonds. I t is concluded that the n-energy of benzylideneaniline, likc that of stilhenti or azobenzene, \\auld favor a planar conformation. The increased torsional aiiglc: in 1 as compared to thc other two iso-conjugatc systems arises from a larger steric interaction between phenyl- and bridgc